Abstract This work is aimed at evaluating the conversion of Pterocarpus indicus twigs into activated carbon via composite chemical activation for methylene blue and congo red dyes adsorption. The activated carbons were prepared by single-step chemical activation using zinc chloride and/or phosphoric acid at different mass impregnation ratios at 600 °C for 90 min. The activated carbons were characterized for textural properties and surface chemistry. The batch adsorption was investigated at different concentrations (5–1,000 mg/L), contact times (2–540 min) and temperatures (30–60 °C). Phosphoric acid-activated twigs carbon showed a high surface area of 1,445 m2/g with maximum methylene blue adsorption capacity of 438 mg/g. On the other hand, a composite-activated carbon yields a 217 mg/g of congo red removal. The adsorption data for both dyes fitted well with Langmuir and pseudo-second-order kinetics models, indicating the predominance of chemical adsorption through monolayer coverage of dye molecules on the homogenous surface of activated carbon. The thermodynamics properties of dye adsorption onto twigs-derived activated carbons indicated that the process is endothermic, spontaneous and favourable at high temperature. Overall, activated carbons derived from Pterocarpus indicus twigs could be effectively used for dye wastewater treatment.